1. Field of the Invention
This invention relates generally to the separation of gaseous mixtures by selective adsorption, and more specifically to a pressure swing adsorption system designed to purify and quantitatively recover a plurality of components from a multicomponent gaseous mixture. Pressure swing adsorption is an art-recognized method which may be viewed as a four-step process such as the following. The adsorption bed is pressurized by entry of gas from the bottom of the bed while the top end of the bed is closed. This is referred to as the pressurization step. The next step is high pressure feed, wherein feed gas enters under pressure from the top of the column and effluent is allowed to escape from the bottom of the column. At the conclusion of this step the column is closed at both ends and the pressurized gas is then released by opening the top end of the column. This is referred to as the blowdown step. After the pressure has been reduced to a predetermined level by blowdown, the column is next purged of remaining product by feeding recycled product gas into the bottom end of the bed and allowing the gas remaining in the column to be forced out of the top end as effluent. This step would normally be terminated at the point where the purging gas reaches the top end of the column. The effluents from the blowdown and purge steps contain the component adsorbed by the column. This is generally referred to as the secondary product of the column. The primary product is the component or components which pass through the bed unadsorbed, i.e., the high pressure feed effluent.
Pressure swing adsorption has been used in numerous applications, including the separation of carbon dioxide and hydrogen from the effluent of a shift convertor in a hydrocarbon reforming plant, and the recovery of hydrogen and methane from the effluent gas of a hydrodesulfurization plant, as discussed below. As normally practiced, the process results in the non-quantitative recovery and purification of a single secondary product, and the marginal enrichment of a primary product. By contrast, complementary pressure swing adsorption, as presently disclosed and claimed, allows for the purification and quantitative recovery of two or more components from the feed gas. This is accomplished by feeding the exhaust from a column selective for one component into another column which is selective for another component.
2. Prior Art
Pressure swing adsorption systems designed for fractionation of gaseous multicomponent mixtures by selective adsorption are well known in the art. See e.g. U.S. Pat. Nos. 3,138,439, 3,142,547, 3,788,037, 3,221,476, 3,430,418, 3,720,042, 3,102,013, 3,149,934, 3,176,444, 3,237,379, 3,944,400, and 4,000,990.
U.S. Pat. No. 4,329,158 discusses the separation of oxygen from air using pressure swing adsorption. The process therein uses a first bed which adsorbs water and carbon dioxide and a second bed for the adsorption of nitrogen, which leaves an oxygen rich primary effluent product.
U.S. Pat. No. 4,264,340 discusses a similar pressure swing adsorption system wherein the secondary nitrogen component collected by the second bed is used to rinse both the first and second beds.
U.S. Pat. No. 4,171,206 discusses a pressure swing adsorption system for the separation of carbon dioxide and hydrogen from the effluent of a shift convertor in a hydrocarbon reforming plant. A first bed adsorbs carbon dioxide, a second bed adsorbs tertiary impurities consisting primarily of carbon monoxide and methane, and hydrogen is recovered as the primary component.
U.S. Pat. No. 4,171,207 discusses the use of pressure swing adsorption for the recovery of hydrogen and methane from the effluent gas of a hydrodesulfurization plant. A first bed adsorbs the C2-C6 hydrocarbon impurities and a second bed adsorbs the methane, leaving the enriched hydrogen component as the primary product.
It is significant that the prior art does not recognize nor teach a process wherein a plurality of columns adsorb each of the several components desired to be purified. Nor does the prior art teach a process wherein the several components to be purified are similar in nature such that a separation by selective adsorption would be difficult to accomplish. The prior art also does not teach a process wherein the product from a given column is used as feed for a complementary column. None of the processes taught by the prior art would be effective in the difficult separations to which the present invention is directed, e.g., complementary adsorption and purification of both nitrogen and oxygen from air. The most that the prior art processes could achieve in this regard would be a purification of one of these components and an enrichment of the other component in a mixed primary product. The prior art does not suggest the unexpectedly high degree of purification and recovery of each of these components achievable by the present process, nor the efficiency with which the present process achieves this result.